This invention pertains to the art of rendering methods and systems. Rendering involves the splitting of organic materials typically of animal origin into its three major component elements comprising water, fat and solid proteinaceous meal. A typical rendering raw material is generally comprised of 25 percent by weight fat, 25 percent solids and 50 percent water. The solids are usually at least 50 percent protein with the remainder comprising bone, minerals and carbohydrates.
Water is a waste material in the rendering process and is usually sewered. Fat and protein are marketable commodities with major uses in a variety of industries. The quality and efficiency of the fat and protein separation are the criteria under which any rendering process is evaluated.
Numerous techniques and processes for rendering have been known and utilized. See U.S. Pat. No. 4,259,252 for a detailed background description of a variety of known rendering methods. However, it has been found that the disadvantages present in most prior rendering methods are such that the methods are of limited economic and practical value.
One known rendering method comprises a dry rendering batch process. By this process a quantity of organic raw material, typically comprising meat by-products, is loaded in a cooker with steam heated jackets and rotors. After the cooker is closed, steam is applied and the charge is heated. Moisture is removed from the batch by venting it as a vapor. When the batch is dry it is dumped. Part of the fat from the batch is drained in a drainage device. The fatty solids from the drainage device are fed to a screw press to remove additional fat. Typically, the residual fat content in the solids subsequent to pressing is in the range from eight to twelve percent. Major disadvantages of this process are low capacity and high operating cost. A single batch cooker may handle a load of five tons of raw materials in two to three hours of operation. Many large rendering plants require capacities of twenty or more tons per hour. Typically, to evaporate one pound of water from the material in the batch cooker requires 1.5 to 2.5 pounds of steam and thus cost of energy to operate a batch system is high. In addition, the use of a screw press to remove fat is particularly expensive both to acquire as capital equipment, and also to operate.
To adapt a batch cooker to high capacity, various manufacturers have developed (notably the Dupps Company of Germantown, Ohio and the Stord-Bartz Company of Bergen, Norway) continuous autoclaves or cookers which allow continuous feeding of raw material and discharging of dried product. While this continuous feed process somewhat alleviated the low capacity problem (continuous cookers may process up to fifteen tons per hour) the cost of energy is still high. In addition, the capital investment and operating costs related to all the steps to separate the fat from the solids are also high. With the mechanical extraction methods prevalent in the industry, it is difficult to achieve residual fat levels of less than ten percent in the solids. As the value of the fat is usually higher than the solids, there is a considerable economic penalty in leaving fat in the meat meal solids.
Another type of continuous rendering method is disclosed in U.S. Pat. No. 4,259,252. In this method the raw material to be rendered is first comminuted and mixed with molten fat. The resulting slurry is subsequently pumped to evaporators to remove moisture. The dry slurry from the evaporator is then centrifuged. Part of the fat separated is returned to slurry more raw material. The solids removed from the centrifuge are then pressed in a conventional manner to remove the residual fat. This system, known commercially as the "C-G system", has a major advantage: high capacity. Systems with capacities of forty to fifty tons per hour have been built in accordance with this method. Another major advantage in the use of evaporators is that they lend themselves to staging, i.e., part of the water may be evaporated in one stage and the vapor from that stage may be conducted to the next evaporator stage to evaporate water in that stage. Thus, energy may be saved by reutilization of the steam energy applied. A corollary advantage of this method is that because the evaporator stages are operated under vacuum, the fat and the meat solids are dried under conditions of low temperature resulting in a quality of the products approaching the intrinsic quality of the components in the raw material fed to the system. However, the fatty solids obtained from the centrifuge must still be treated to remove residual fat. According to this method it is not possible to centrifuge a dried slurry of meat solids to form a cake with less than 30 to 35 percent residual fat. A screw press is most often used to reduce the residual fat level to about ten to twelve percent. The fat from the centrifuge and from the press usually contains a fairly high percentage (such as 2.0 percent or more by weight) of finely divided insoluble solids ("fines") which must be removed by means of a filter press or a polishing centrifuge to reduce the fines to an acceptable level. An acceptable level would be 0.05 to 0.1 percent by weight dependent upon the grade of fat. A substantial problem with this method is that the press and centrifuge represent major capital investments and additional operating cost.
The systems described above are called "dry" methods in the trade because no water is added to the raw material to assist in the extraction of fat. There are also several variations of "wet" rendering methods.
In wet rendering methods hot water is added to the raw material to extract the fat. In theory, this method produces a protein solids material which is very low in fat (substantially below what can be achieved with the mechanical extraction methods such as the screw pressing discussed above in dry rendering methods). Typically wet rendering will produce protein solids of six or seven percent by weight residual fat instead of in excess of ten percent as in the dry rendering methods.
The wet systems have the advantage of relatively low residual fat in the meat meal. The disadvantages of such systems are the extensive and expensive treatment of the stick water that is required and the problems encountered in drying the defatted solids. Stick water is the water that is separated from the fats and solids in a wet rendering method and it contains substantial quantities of water soluble proteins. With certain types of raw materials and under certain operating conditions, the protein solids are difficult to dry. These materials tend to be sticky and adhere to heat transfer surfaces during the drying operation.
Often a centrifuge is employed in the wet rendering method. The liquid stream feeding it consists of water, fat and proteins, both soluble and insoluble, and is therefore often equally difficult to handle. The presence of proteins promotes the formation of emulsions which inhibit the complete separation of the fat from the liquid stream. The stick water separated in the centrifuge would contain in addition to insoluble and soluble proteins appreciable quantities of fat. In addition, the quantity of stick water is to be handled is typically very large. The raw material usually contains at least 50 percent water by weight and to extract the fat substantial additional water is needed. In a large plant, large quantities of stick water require a major capital investment in evaporators and/or water pollution control systems. The cost of operating these systems is high. Because of these problems, wet rendering is no longer a commercially significant process in the United States and Canada, where approximately half of the world's rendering production takes place.
The present invention contemplates a new and improved rendering process which overcomes all the above referred to problems and others not discussed to provide a new pretreatment and rendering process which is simple in design, economical to build and operate, readily adaptable to a plurality of types of materials to be rendered and which provides improved fat and protein solid products at a lower cost and with improved efficiency.